Hexagonal Ba ferrite and Sr ferrite are known in the prior art as magnetic materials for use in sintered ferrite magnets. In recent years, magnetoplumbite (M-type) Ba ferrite and Sr ferrite have become the most commonly used ferrites. M-type ferrite is represented by the general formula AFe12O19, with Ba or Sr as the element represented by A. M-type ferrites, wherein the element represented by A is Sr with a portion thereof being replaced with a rare earth element and a portion of the Fe being replaced with Co, are known to exhibit excellent magnetic properties including residual flux density and coercive force (for example, see Patent documents 1 and 2). Such M-type ferrites must contain La as the rare earth element. This is because La is the rare earth element that has the highest solubility limit for hexagonal M-type ferrite. Patent documents 1 and 2 also teach that using La as a substitute element for the element represented by A can increase solubility of the Co which is substituting for a portion of the Fe, thus enhancing the magnetic properties as a result.
When the element represented by A in the general formula shown above is Ca which has a smaller ion radius than either Sr or Ba, the crystal structure of hexagonal ferrite is not exhibited and the material cannot be used as a magnetic material. Even when the element represented by A is Ca, however, replacing a portion thereof with La can result in the crystal structure of hexagonal ferrite. Also, it is known that replacing a portion of the Fe with Co causes the sintered ferrite magnet to exhibit a high magnetic property (see Patent document 3). In other words, the magnetic material is M-type ferrite wherein the element represented by A is Ca and a portion thereof is replaced with a rare earth element containing La as an essential component, while a portion of the Fe is replaced with Co.
M-type ferrites wherein the element represented by A is Ca are disclosed in Patent documents 4 and 5 as well as in Patent document 3 cited above. Patent document 4 discloses an oxide magnetic material which comprises, as the main phase, ferrite having a hexagonal M-type structure represented by the following formula (9), with the aim of providing an oxide magnetic material and sintered magnet that improve the residual flux density (Br) and coercive force (HcJ) while also exhibiting a high squareness ratio.(1−x)CaO·(x/2)R2O3·(n−y/2)Fe2O3·yMO:   (9)In formula (9), R is at least one element selected from among La, Nd and Pr and definitely includes La, M is at least one element selected from among Co, Zn, Ni and Mn and definitely includes Co, and the letters x, y and n representing the molar ratios are such satisfying the inequalities 0.4≦x≦0.6, 0.2≦y≦0.35 and 4≦n≦6 and the relational expression 1.4≦x/y≦2.5.
Also, Patent document 5 proposes a sintered ferrite magnet having a composition represented by the following general formula (10), with the aim of achieving high coercive force that is not reduced even with small thicknesses, while also maintaining high residual flux density.A1−x−y+aCax+yRy+cFe2n−zCoz+dO19:   (10)In formula (10), element A represents Sr or Sr and Ba, element R is at least one rare earth element including Y and definitely includes La, while x, y, z and n respectively represent the elements Ca and R and the content and molar ratio of Ca in the calcined body, and a, b, c and d respectively represent the amounts of element A, elements Ca and R and Co which are added during the milling step for the calcined body, with the following conditions:0.03≦x≦0.4, 0.1≦y≦0.6, 0≦z≦0.4, 4≦n≦10, x+y<1, 0.03≦x+b≦0.4, 0.1≦y+c≦0.6, 0.1≦z+d≦0.4, 0.50≦[(1−x−y+a)/(1−y+a+b)]≦0.97, 1.1≦(y+c)/(z+d)≦1.8, 1.0≦(y+c)/x≦20, 0.1≦x/(z+d)≦1.2being satisfied.    [Patent document 1] Japanese Unexamined Patent Publication HEI No. 11-154604    [Patent document 2] Japanese Unexamined Patent Publication No. 2000-195715    [Patent document 3] Japanese Unexamined Patent Publication HEI No. 12-223307    [Patent document 4] Japanese Unexamined Patent Publication No. 2006-104050    [Patent document 5] International Patent Publication No. WO 2005/027153